The present invention relates to a microporous membrane of a polyolefin composition containing an ultra-high-molecular-weight polyolefin, and a method for efficiently producing such a microporous membrane.
Microporous membranes are widely used in various applications such as battery separators, electrolytic capacitor separators, various filters, moisture-permeable, waterproof clothes, reverse osmosis membranes, ultrafiltration membranes, microfiltration membranes, etc.
Microporous polyolefin membranes are conventionally produced by various processes. One example of such processes is an extraction process comprising the steps of mixing a polyolefin with a pore-forming agent such as fine powder of different polymers in such a manner as to achieve micro-dispersion and subsequently extracting the dispersed pore-forming agent. Another process is a phase separation process by which polyolefin is divided into fine phases by a solvent, thereby forming a porous structure. Further, there is a drawing process comprising the steps of forming a polyolefin article containing solid fillers finely dispersed therein and imparting a strain to the article by drawing to break the interfaces between the polymer phase and the solid fillers, thereby forming pores in the article. In these processes, however, polyolefins having a molecular weight lower than 500,000 are usually used, so that the thinning and strengthening of membranes by drawing are limited.
Recently, there has been developed an ultra-high-molecular-weight polyolefin which can be formed into a high-strength, high-modulus membrane. With this development, there have been proposed various processes for producing a high-strength microporous membrane from it.
One of such processes is disclosed in, for example, Japanese Patent Laid-Open No. 58-5228. According to this process, an ultra-high-molecular-weight polyolefin is dissolved in a nonvolatile solvent and the resulting solution is made into a gel in the form of fiber or membrane. The solvent-containing gel is subjected to an extraction treatment with a volatile solvent and then stretched while heating. This process is, however, disadvantageous in that the gel cannot be biaxially oriented at a high draw ratio because it has a porous structure highly swollen with a nonvolatile solvent. The resulting membrane has a low strength and a large pore diameter on account of its reticulate structure which easily expands and breaks. Another disadvantage of this process is that the gel in the form of a membrane is liable to warpage because of uneven evaporation of the volatile solvent. Further, it cannot be subjected to orientation at a high draw ratio because of the shrinkage and compaction of the reticulate structure of the gel which take place after the extraction of the nonvolatile solvent by a volatile solvent.
Various attempts have been proposed to produce a microporous membrane of an ultra-high-molecular-weight polyolefin (polyethylene) by forming a gel-like sheet from a heated solution of an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5.times.10.sup.5 or more, controlling a solvent amount in the gel-like sheet by a solvent-removing treatment, and then stretching it while heating thereby removing the remaining solvent.
Japanese Patent Laid-Open No. 60-242035 discloses a process for producing a microporous ultra-high-molecular-weight polyethylene membrane having a thickness of 10 .mu.m or less, a breaking strength of 200 kg/cm.sup.2 or more, and a void volume of 30% or more by dissolving ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 5.times.10.sup.5 or more in a solvent while heating, forming a gel-like sheet from the resulting solution, removing a solvent from the gel-like sheet until the solvent content decreases to 10-80 weight %, and then stretching the sheet while heating, thereby removing a residual solvent.
Japanese Patent Laid-Open No. 61-195132 discloses a method of producing a microporous membrane by forming a gel-like article from a solution of an .alpha.-olefin polymer having a weight-average molecular weight of 5.times.10.sup.5 or more, removing at least 10 weight % of a solvent from the gel-like article so that the .alpha.-olefin polymer content in the gel-like article becomes 10-90 weight %, stretching it at a temperature equal to or lower than a melting point of the .alpha.-olefin polymer+10.degree. C., and removing the remaining solvent from the resulting stretched article.
Japanese Patent Laid-Open No. 61-195133 discloses a microporous membrane made of an .alpha.-olefin polymer having a weight-average molecular weight of 5.times.10.sup.5 or more and having an average pore diameter of 0.001-1 .mu.m and a porosity of 30-90%, which is stretched two times or more in one direction at an areal stretching ratio of 20 times or more.
Japanese Patent Laid-Open No. 63-39602 discloses a method of producing a microporous polyethylene membrane having a pure water permeability of 100 l/m.sup.2 .multidot.hr.multidot.atm or more and a .gamma.-globulin block ratio of 50% or more, which comprises forming a gel-like article from a solution of polyethylene having a weight-average molecular weight of 5.times.10.sup.5 or more, removing a solvent from the gel-like article so that the solvent content in the gel-like article becomes more than 80 weight % and 95 weight % or less, stretching it two times in one direction and 20 times or more in an areal ratio at a temperature of 120.degree. C. or lower, and removing the remaining solvent from the resulting stretched article. This microporous polyethylene membrane is excellent in water permeability and suitable for separating proteins, etc. because of its fine pores.
Further, Japanese Patent Laid-Open No. 63-273651 discloses a method of producing a microporous membrane by preparing a solution of an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5.times.10.sup.5 or more, extruding the solution through a die while rapidly cooling the solution to a gelation temperature or lower, so that an ultra-high-molecular-weight polyolefin content in the gel-like article becomes 10-90 weight %, stretching it at a temperature equal to or lower than a melting point of the ultra-high-molecular-weight polyolefin+10.degree. C., and then removing the remaining solvent. This method can provide a microporous membrane having a thickness of 10 .mu.m or more, which is suitable for applications requiring large strength and pressure resistance.
However, in any of the above methods, since the ultra-high-molecular-weight polyolefin is biaxially oriented, a diluted solution of polyolefin should be prepared. Accordingly, the ultra-high-molecular-weight polyolefin solution suffers from large swelling and neck-in at the exit of a die, resulting in the difficulty of sheet formation. Further, since the resulting sheet contains an excess amount of a solvent, mere stretching fails to provide a desired microporous membrane. Accordingly, a solvent-removing treatment is necessary to control the solvent content in the sheet before stretching, meaning that the productivity of a microporous membrane is relatively low.